Most domestic refrigerators have a single refrigeration cycle serving both freezer and fresh food compartments, that is, compartments maintained at two different temperatures. This low-cost approach, while conserving capital expense, is not efficient from a thermodynamic point of view. As the refrigeration cycle must deliver all cooling at 0.degree. F., while the food compartment is generally maintained at a much higher temperature, e.g., 35.degree.-40.degree. F., the compressor works harder, and consumes more electricity than necessary for cooling of the food compartment.
U.S. Pat. No. 4,910,972 provides an improvement on the conventional cycle. This system, designed for use with a single refrigerant, R12, provides a cycle in which the refrigerant leaving the condenser passes a first expansion device and a high-temperature evaporator. In this HTE, half the refrigerant is evaporated, which provides cooling for the refrigerator, or high-temperature compartment. The vapor is separated from the liquid in a phase separator, and the liquid is passed through a second expansion device to a low-temperature evaporator, which provides cooling for the freezer at 0.degree.-5.degree. F. The vapor from the LTE is compressed first in a low-stage compressor, and then mixed with the suction vapor from the high-temperature evaporator, in a second compressor. Analysis indicates an improvement of 35.5% using R12 as the refrigerant.
A further improvement is described in U.S. Pat. No. 4,918,942. In this patent, heat exchanges are added which exchange heat between liquid streams upstream of expansion valves with their respective suction vapors. This further improves the system. Thus, in comparison to the basic design in U.S. Pat. No. 4,910,972, showing a 35.5% improvement in comparison to a single-stage refrigeration cycle, the system of U.S. Pat. No. 4,918,942 gives a 48.1% improvement over the same conventional single system, a significant improvement beyond the two-stage cycle of the '972 patent.
Further improvements are desired. Particularly, improvements which take advantage of the operational characteristics of mixed refrigerant systems are particularly promising.